Gas sensor, especially a lambda probe

ABSTRACT

A gas sensor has improved electrical properties. In the top view of the layer planes of the body of the sensor, printed circuit traces for electrodes are arranged outside of cavities in the body, e.g., outside of the reference air duct. Furthermore, the electrodes are enlarged in the direction of the exhaust-side end of the sensor. Also, the printed circuit traces have an increased layer thickness or are formed as a double layer.

FIELD OF THE INVENTION

The present invention relates to gas sensors, e.g., lambda probes,including a body formed as a sintered ceramic laminate and a referenceair duct situated therein within a layer of the laminate, an electricalresistance heater provided on its one side and an electrodeconfiguration provided on its other side, the electrode configurationincluding at least one reference electrode that:is arranged on theinside of a boundary wall of the reference air duct and is permeable forgases at least regionally and that includes a Nernst electrode that isacted upon by gases to be sensed, the Nernst electrode is also at leastregionally permeable for gases and separate from the reference electrodeby a solid electrolyte layer that is conductive and permeable for ions,e.g., oxygen ions, and the electrodes connected to printed circuittraces that essentially extend in the direction of the reference airduct.

BACKGROUND INFORMATION

Exhaust systems of modern internal combustion engines, particularly formotor vehicles, are regularly provided with catalytic converters forconverting harmful exhaust gases into harmless reaction products. Inorder for the catalytic converters to function well, it is necessary tofeed air and fuel to the engine in a predefined proportion. The enginecontrols provided for this purpose are connected on their input side toa so-called lambda probe the signals of which represent the compositionof the exhaust gas and, thus, enable the engine control to adjust theratio of fuel and combustion air in a manner optimal for the catalyticconverter.

Two configurations are conventional in this connection.

In the one configuration, stoichiometric combustion is targeted, i.e.,the oxygen quantity in the combustion air corresponds exactly to theoxygen requirement for complete combustion of the supplied fuel.Therefore, the engine is operated using neither an excess of oxygen(λ>1) nor using a deficiency of oxygen (λ>1). This operating method may,therefore, by characterized by λ=1.

When sensing exhaust gas,: narrow-band lambda probes where the Nernstelectrode is acted upon by the exhaust gas as directly as possible aresufficient for this stoichiometric combustion.

In this instance, the effect is used by the engine control that anelectrical voltage able to be tapped off between the reference electrodeand the Nernst electrode and generated by diffusion of oxygen ionssignificantly changes its value in the vicinity of λ=1, and a signal isconsequently available that clearly displays a deviation from thedesired operating mode using stoichiometric combustion in the directionof an operating mode having an oxygen deficiency as well as in thedirection of an operating mode having an oxygen excess.

Such sensors are described in German Published Patent Application No. 4401 749, for example.

In the other configuration, the objective is predominant operation ofthe internal combustion engine with an oxygen excess during combustionsince the fuel consumption is able to be noticeably reduced as a result.However, during combustion using an oxygen excess, harmful nitrogenoxides are produced that may only be absorbed for a limited time byso-called adsorption catalysts in the exhaust branch of the motorvehicle. In each case prior to exhausting the absorption capacity of theadsorption catalysts, the engine operation must be switched over brieflyto combustion with an oxygen deficiency in order for the incompletelycombusted fuel components now entering the exhaust branch to be able toreduce the nitrogen oxides previously stored in the catalytic converterto nitrogen. In this instance, the engine control, i.e., the internalcombustion engine, must be constantly switched at intervals between anoperating mode that is predominant with respect to time and in which thevalues of λ are greater than 1 and a relatively brief operating mode inwhich the values of λ are less than 1.

Broadband lambda probes are necessary for such an intermittent operatingmode having drastically changing values of λ.

In the case of such lambda probes, the Nernst electrode is arranged at aseparate chamber that communicates with the exhaust-gas stream via adiffusion path arranged in the body of the probe. Arranged within thischamber is also an internal pump electrode that may be electricallyconnected to the Nernst electrode and also cooperates through a solidelectrolyte layer with an external pump electrode that is exposed to theexhaust-gas stream as directly as possible. If an external electricalvoltage is applied between the two pump electrodes, which are bothconfigured to be permeable for gases at least regionally, an oxygen ioncurrent the direction of which depends on the polarity of the appliedvoltage and the intensity if which is determined by the electricalvoltage difference as well as by the difference in the oxygenconcentration at the pump electrodes is generated between the pumpelectrodes. This oxygen ion current accordingly controls the diffusioncurrent of the exhaust gases in the diffusion chamber. The externalelectrical voltage between the pump electrodes and the electricalcurrent occurring between the pump electrodes due to the oxygen ioncurrent are adjusted by a controller so that an electrical voltagehaving a predefined setpoint value is always maintained between thereference electrode and the Nernst electrode. As such, the polarity andintensity of the electrical current occurring between the pumpelectrodes are a signal that correlates to the composition of theexhaust gases and, thus, to the λ values.

Such probes are described in German Published Patent Application No. 3744 206, for example.

Aging processes such as pollution change the properties of theaforementioned gas sensors.

SUMMARY

In accordance with the present invention, it is provided that in a topview of the layer planes of the laminate, the printed circuit tracescorresponding to the electrodes are arranged at least partially next tothe reference air duct.

This may be applicable for the printed circuit traces of the pumpelectrodes.

The present invention is based on using the pressing pressure exertedduring and/or prior to sintering the probe body to the laminate forcompressing the composite structure of the printed circuit traces and inthis connection to effectively increase the pressure forces exerted onthe printed circuit traces by arranging the printed circuit traces inthe laminate without being covered by hollow spaces as viewed fromabove. Consequently, a smaller electrical resistance of the printedcircuit traces as well as a higher durability of the printed circuittraces with respect to aging effects is achieved with the result thatthe change in the electrical properties of the probe in the case ofincreasing age are significantly reduced.

In addition or alternatively, further measures may be provided. Forexample, the internal and/or external pump electrode may have a surfacethat is larger than the base plan of the gas chamber arranged betweenthe Nernst electrode and the internal pump electrode.

The pump electrodes on a region diametrically opposed to thecorresponding printed circuit trace, i.e., in the direction of the topend of the probe body, may extend beyond the base plan of the gaschamber.

Furthermore, it may be advantageous for constant electrical propertieswhen the printed circuit traces have a comparably large layer thickness.For this purpose, the printed circuit traces may be produced usingprinting technology with relatively wide-meshed screens (e.g., screensincluding a 250 mesh). Moreover, the printed circuit traces may also beprinted as a double-layer.

Finally, the pressure material for the printed circuit traces may have ahigh proportion of electrically conductive particles, e.g., based onplatinum.

Example embodiments of the lambda probe according to the presentinvention are explained in greater detail below and are illustrated inthe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a broadband lambda probecorresponding to line of intersection I—I illustrated in FIG. 2 and FIG.3 in the region of the top end of the probe body projecting into theexhaust-gas stream.

FIG. 2 is a top view corresponding to arrow II illustrated in FIG. 1 ofsolid electrolyte layer.

FIG. 3 is a top view corresponding to arrow III illustrated in FIG. 1 ofdifferent layers of the laminate including electrodes as well ascorresponding printed circuit traces.

FIG. 4 is a longitudinal cross-sectional view of part of the probecorresponding to line of intersection IV—IV illustrated in FIG. 1.

DETAILED DESCRIPTION

As illustrated in FIG. 1, the lambda probe includes a body 1, which isformed as a ceramic laminate. The layers of the laminate are placed ordeposited on one another in the green condition. Subsequent sintering,which may be performed after or during simultaneous pressing of thelaminate, produces a hard ceramic body 1.

In the example embodiment illustrated in FIG. 1, a bottom layer 2 isprovided in the form of a thicker film of zirconium oxide. Above this isan electrically insulating double layer 3, in which an electricalresistance heater 4 as well as corresponding printed circuit traces forelectrical current supply are embedded. Above that is layer 5, which isproduced and patterned by screen printing and is made, for example, of azirconium oxide paste. Recessed within this layer is a reference airduct 6, the base plan of which is illustrated by manner of example inFIG. 2 and is explained in greater detail below. As illustrated, thisreference air duct 6 may have two end regions 6′, which communicate withone another, in the area of the sectional plane illustrated in FIG. 1.

In some instances, layer 5 may also be formed by a film in which duct 6is punched out.

Above layer 5 is a solid electrolyte layer 7, e.g., in the form of afilm made of zirconium oxide to which yttrium oxide is added. Agas-permeable, layered reference electrode 8 of a porous platinummaterial, which is connected via a layered printed circuit trace 8′connected thereto (see FIG. 2) to a connection contact on body 1explained below, is arranged on the side of layer 7 facing reference airduct 6, i.e., between layers 5 and 7, at least in the area of endregions 6′ of reference air duct 6.

Above solid electrolyte layer 8 is a thin layer 9, which is patternedusing printing technology and is in turn produced from a zirconium oxidepaste, for example. This layer 9 includes a large recess that isarranged centrically to an exhaust-gas access hole 10, which extendsthrough body 1 perpendicularly to its layers. Porous material 12 isdeposited within the indicated recess while leaving open a ring space11. As illustrated, access hole 10 may be configured as a blind hole oras an opening passing completely through body 1.

In the region of annular space 11, solid electrolyte layer 7 supports agas-permeable, layered Nernst electrode 13 of a porous platinummaterial.

Another solid electrolyte layer 14, e.g., in the form of a film made ofzirconium oxide to which yttrium oxide is added, is above layer 9 andporous material 12. This layer 14 supports on it side facing annularspace 11 as well as on its side away from annular space 11gas-permeable, internal and external pump electrodes 15 and 16 made ofan at least regionally porous platinum material, these electrodes 15 and16 are formed such that in a top view of the layers of body 1, they atleast essentially cover annular space 11. A gas-permeable protectivelayer 17 is above layer 14.

In some instances, a positive image of reference air duct 6 as well asof its end pieces 6′ and orifices 6″ may also be imprinted on layer 7using a material that is disintegrated or burned off when sintering body1 or forms a porous, effectively gas-permeable structure.

In general, it may be possible to print layer 3 in a mirror image tolayer 7 using the material of layer 5 and, in some instances, also usingthe material provided for the positive image of reference air duct 6 andit parts 6′ and 6″. In this manner, layer 5 is able to be produced witha greater thickness.

The previously described lambda probe functions as follows:

The end of body 1 including exhaust-gas access hole 10 is arranged inthe exhaust-gas stream or in a region communicating with the exhaust-gasstream of an internal combustion engine, while the other end of body 1is acted upon by reference air typically from the atmosphere.

Reference air reaches end pieces 6′ of the reference air duct viareference air duct 6 and its orifices 6″. Via exhaust-gas access hole10, exhaust gas reaches porous material 12, through which the exhaustgas diffuses into annular space 11.

When the exhaust gas-side end of body 1 is sufficiently heated byelectrical resistance heater 4, an electrical voltage, the magnitude ofwhich depends on the partial oxygen pressures within end pieces 6′ ofthe reference air duct as well as within annular space 11, is able to betapped off between reference electrode 8 and Nernst electrode 13 andconsequently between plated through-holes 19 and 20. In this context,the effect is taken advantage of that solid electrolyte layer 7 conductsoxygen ions and the platinum material of aforementioned electrodes 8 and13 promotes or enables the formation of these oxygen ions with theresult that an electrical potential difference that is dependent on thepartial oxygen pressure at electrodes 8 and 13 and results in acorresponding ion migration occurs in solid electrolyte layer 7. Thispotential difference is also referred to as the Nernst voltage.

The partial oxygen pressure in annular space 11 is able to be controlledin that an external electrical voltage having controllable polarity isapplied between pump electrodes 15 and 16. The corresponding voltagesource is connected to plated through-holes or contacts that areelectrically connected to pump electrodes 15 and 16.

In this instance, the effect is in turn used that the platinum materialof electrodes 15 and 16 results in the formation of oxygen ions and anoxygen ion current flowing through solid electrolyte layer 14 and havingan intensity and direction dependent on the electrical voltage and itspolarity is then produced by the external electrical voltage betweenelectrodes 15 and 16. Thus, an electrical signal is able to be tappedoff between pump electrodes 15 and 16, e.g., is able to be determined byascertaining the voltage and current intensity of the electricalresistance of the electric circuit leading across the pump electrodes.

The electrical voltage and consequently also the electrical currentbetween pump electrodes 15 and 16 are controlled by a controller suchthat electrical voltage able to be tapped off between referenceelectrode 8 and Nernst electrode 13 and consequently the partial oxygenpressure in annular space 11 always correspond to a defined setpointvalue. Therefore, the electrical current able to be tapped off betweenpump electrodes 15 and 16 is a measure of the oxygen content of theexhaust gas relative to the reference air.

When external pump electrode 16 is at an electrically positive potentialwith respect to internal pump electrode 15, operating conditions whereλ>1 exist. In the case of a reverse polarity, operating conditions ofλ<1 exist, the magnitude of the electrical resistance between electrodes15 and 16 correlating to the magnitude of λ.

The values of λ may be acquired in a large value range.

In the case of the narrow-band lambda probe, external protective layer17 is above Nernst electrode 13, i.e., layers 9 and 14 as well as pumpelectrodes 15 and 16 may not be necessary in comparison with thearrangements illustrated in FIGS. 1 and 3. Given a known and constantpartial oxygen pressure, the electrical voltage able to be tapped offbetween electrodes 8 and 13 is a measure of the partial oxygen pressureof the exhaust gases.

FIG. 2 is a top view corresponding to arrow II illustrated in FIG. 1 ofsolid electrolyte layer 7. Reference electrode 8 as well as a printedcircuit trace 8′ connected thereto are imprinted on the side of thislayer 7 illustrated in FIG. 2. This printed circuit trace 8′ leads to aplated through-hole, which is able to extend, for example, through layer7 as well as the layers above layer 7 illustrated in FIG. 1 and toelectrically connect printed circuit trace 8′ to a contact tag arrangedexternally on body 1 on its reference air-side end.

Furthermore, a dashed line illustrated in FIG. 2 represents the positionof reference air duct 6 including its ends 6′ arranged in the region ofreference electrode 8.

As illustrated in FIG. 2, printed circuit trace 8′ is outside ofreference air duct 6. Consequently, printed circuit trace 8′ issubjected to an increased pressing pressure when the laminate of body 1is pressed prior to and/or during sintering in order to ensure goodcohesion of the layers of the laminate.

While reduced pressing pressures occur during this pressing above and/orbelow reference air duct 6 because almost no forces are able to beapplied over the hollow space of duct 6, a high pressure may always beensured in regions next to reference air duct 6 because in this instancethere are no considerable cavities in the laminate.

The aforementioned increase in pressing pressure at printed circuittrace 8′ is especially effective when reference air duct 6 is producedwithin a film-like layer 5 by punching.

It may also be provided that instead of fork-shaped end regions 6′ ofthe reference air duct, only one single end piece enlarged in someinstances with respect to the rest of reference air duct 6 be arrangedbelow gas access opening 10, which in this case may be configured as ablind hole in order to be able to ensure a separation from reference airduct 6. Reference electrode 8 has a form similar to the aforementionedend piece of the reference air duct such that reference electrode 8covers the aforementioned end piece of reference air duct 6 with more orless excess.

FIG. 3 is a top view of solid electrolyte layer 14 corresponding toarrow III illustrated in FIG. 1. Dotted lines represent the position ofannular space 11 as well as of porous material 12 via which annularspace 11 communicates with gas access hole 10.

External pump electrode 16 is imprinted on the side of solid electrolytelayer 14 illustrated in FIG. 3. It has an ring-shaped configurationsimilar to the ring shape of annular space 11. However, external pumpelectrode 16 may be significantly enlarged, e.g., in the direction ofthe exhaust-side end of layer 14 and also may project in the directionof the longitudinal sides of layer 14 beyond the borders of annularspace 11.

As a result of this configuration, a good pump effect is able to beachieved already at a low voltage between pump electrodes 15 and 16, itmay be ensured at the same time that a clear proportionality resultsbetween the lambda values and the pump current.

Internal pump electrode 15 may have a shape similar to external pumpelectrode 16.

Moreover, it may be provided for porous material 12 to be arranged inregions adjacent to large-area zones of internal and external pumpelectrodes 15, 16, respectively, having a narrower width in the radialdirection to gas access opening 10. In this manner, the gas access toannular chamber 11 in zones in which pump electrodes 15 and 16 have anincreased pump effect is made easier.

It may be provided for all electrodes to configure the correspondingprinted circuit traces with good electrical conductivity.

For example, this may be achieved in that the material used for printingthe printed circuit traces includes an increased platinum content or anincreased content of other effectively electrically conductiveparticles. While the electrodes may be permeable for gas and aretherefore produced using printing technology with a particle mixturethat, during sintering, forms an electrically conductive layer that ispermeable for gases and ions, gas permeability may not need to beensured for the printed circuit traces. Accordingly, the metal contentof the material of the printed circuit traces may be increased. Forexample, the electrode material may contain a high proportion ofzirconium oxide in comparison with the platinum content, while thematerial of the corresponding printed circuit traces includes a lowzirconium oxide content in comparison with the platinum proportion.

A further possibility for increasing the electrical conductivity of theprinted circuit traces is to provide an increased layer thickness of theprinted circuit traces. This may be achieved, for example, in that theprinted circuit traces are produced by screen printing using comparablywide-meshed nets.

Furthermore, there is the possibility of producing printed circuittraces in each case as a double layer, i.e., two conductive layers maybe arranged or printed one above the other.

FIG. 4 is a longitudinal cross-sectional view corresponding to line ofintersection IV—IV illustrated in FIG. 2 through layers 7, 9, and 14.

Nernst electrode 13, which is arranged on layer 9 and above chamber 11and the form of which may be similar to pump electrodes 15 and 16 asviewed from above, includes an assigned printed circuit trace 13′, whichborders layer 9 and is coincident with printed circuit trace 15′ ofinternal pump electrode 15, so that printed circuit traces 15′ and 13′form a double layer.

To enable such a configuration, layer 14 may first be coated on itsbottom side illustrated in FIG. 4 only right of a border G with thematerial of layer 9. Electrode 15 and corresponding printed circuittrace 15′ are subsequently printed, and a connection between printedcircuit trace 15′ on layer 9 and electrode 15 on layer 14 isautomatically created in the region of border G.

Porous material 12 as well as the still missing part of layer 9 are thenimprinted or superposed on layer 14.

Pump electrode 13 as well as corresponding printed circuit trace 13′ maybe imprinted on layer 7, which is subsequently placed on layer 9, andprinted circuit traces 15′ and 13′ are sintered together when thelaminate is later sintered.

Deviating from the arrangements illustrated in FIG. 3 and 4, e.g., whenreference air duct 6 is produced within layer 5 by punching, printedcircuit traces (e.g., 13′, 15′, 16′) of all electrodes are arrangedoff-center on the appropriate layers of the laminate such that, in thetop view of the layer planes, no covering by reference air duct 6 isable to occur, and pressing the laminate makes it possible to compactthe material of the printed circuit traces effectively while improvingthe electrical conductivity as was explained above by manner of examplewith reference to FIG. 2 for printed circuit trace 8′ of referenceelectrode 8.

What is claimed is:
 1. A gas sensor, comprising: a body configured as asintered ceramic laminate; a reference air duct arranged within a layerof the laminate; an electrical resistance heater arranged on a firstside of the reference air duct; an electrode arrangement arranged on asecond side of the reference air duct, the electrode arrangementincluding at least one internal reference electrode arranged on a borderwall of the reference air duct and at least regionally permeable to gasand a Nernst electrode configured to be acted upon by the gas to besensed and at least regionally permeable to gas, the Nernst electrodeseparated from the reference electrode by a solid electrolyte layerconductive and permeable to ions; and reference and Nernst electrodeprinted circuit traces connected to the respective electrodes, theprinted circuit traces extending essentially in parallel to thereference air duct and, in a top view of layer planes of the laminate,the reference electrode printed circuit trace arranged at leastpartially next to the reference air duct, wherein the printed circuittraces connected to the electrodes have increased electricalconductivity with respect to the electrodes.
 2. The gas sensor accordingto claim 1, further comprising: a separate chamber, the Nernst electrodearranged in the separate chamber; a diffusion path arranged in the body,the Nernst electrode configured to communicate with an exhaust gasstream via the diffusion path; an external pump electrode directlyexposed to the exhaust-gas stream; an internal pump electrode arrangedwithin the separate chamber and configured to cooperate via the solidelectrolyte layer with the external pump electrode; and wherein, in atop view of the layer planes of the laminate, at least one of the Nernstelectrode, the internal pump electrode, and the external pump electrodeextends beyond borders of the chamber.
 3. The gas sensor according toclaim 2, further including an exhaust gas annular space, wherein outsideof the chamber, in a top view, at least one of the Nernst electrode, theinternal pump electrode and the external pump electrode has a large-arearegion that extends beyond the exhaust gas annular space in a directionof an exhaust-side, top end of the body.
 4. The gas sensor according toclaim 1, wherein the gas sensor is configured as a lambda probe.
 5. Thegas sensor according to claim 1, wherein the electrolyte layer isconductive and permeable to oxygen ions.
 6. The gas sensor according toclaim 1, wherein the reference electrode printed circuit trace runsalongside the reference air duct.
 7. A gas sensor, comprising: a bodyconfigured as a sintered ceramic laminate; a reference air duct arrangedwithin a layer of the laminate; an electrical resistance heater arrangedon a first side of the reference air duct; an electrode arrangementarranged on a second side of the reference air duct, the electrodearrangement including at least one internal reference electrode arrangedon a border wall of the reference air duct and at least regionallypermeable to gas and a Nernst electrode configured to be acted upon bythe gas to be sensed and at least regionally permeable to gas, theNernst electrode separated from the reference electrode by a solidelectrolyte layer conductive and permeable to ions; reference and Nernstelectrode printed circuit traces connected to the electrodes, theprinted circuit traces extending essentially in parallel to thereference air duct and, in a top view of layer planes of the laminate,the reference electrode printed circuit trace arranged at leastpartially next to the reference air duct; a separate chamber, the Nernstelectrode arranged in the separate chamber; a diffusion path arranged inthe body, the Nernst electrode configured to communicate with an exhaustgas stream via the diffusion path; an external pump electrode directlyexposed to the exhaust-gas stream; and an internal pump electrodearranged within the separate chamber and configured to cooperate via asecond solid electrolyte layer with the external pump electrode;wherein, in a top view of the layer planes of the laminate, a portion ofat least one of the Nernst electrode, the internal pump electrode, andthe external pump electrode extends beyond borders of the chamber, theportion excluding circuit traces of the at least one of the Nernstelectrode, the internal pump electrode, and the external pump electrode.8. The gas sensor according to claim 7, wherein the printed circuittraces connected to the electrodes have increased electricalconductivity with respect to the electrodes.
 9. The gas sensor accordingto claim 8, wherein the printed circuit traces have one of an increasedmetal and increased platinum content with respect to the electrodes. 10.The gas sensor according to claim 9, wherein the gas sensor isconfigured as a lambda probe.
 11. The gas sensor according to claim 9,wherein the electrolyte layer is conductive and permeable to oxygenions.
 12. The gas sensor according claim 8, wherein the printed circuittraces are configured with an increased thickness with respect to theelectrodes.
 13. The gas sensor according to claim 12, wherein the gassensor is configured as a lambda probe.
 14. The gas sensor according toclaim 12, wherein the electrolyte layer is conductive and permeable tooxygen ions.
 15. The gas sensor according to claim 8, wherein theprinted circuit traces are formed as a double layer.
 16. The gas sensoraccording to claim 15, wherein the gas sensor is configured as a lambdaprobe.
 17. The gas sensor according to claim 15, wherein the electrolytelayer is conductive and permeable to oxygen ions.
 18. The gas sensoraccording to claim 7, wherein the gas sensor is configured as a lambdaprobe.
 19. The sensor according to claim 7, wherein the electrolytelayer is conductive and permeable to oxygen ions.